In all galvanic secondary elements, i.e. accumulators or storage cells, the charging time is a function of the thickness of the active mass. The charging time is the time required for transforming the active mass from its discharged state to its charged state by the electrochemical process.
The thinner the active mass is, the more rapidly can the charging process be effected. However, as the thickness of the active mass increases, the charging process becomes slower. The reason for this is the limited velocity of the ion migration and the resulting limiting current density in amperes/ m.sup.2 which can be developed. As the driving potential or applied voltage during charging increases above a predetermined level, the desired electrochemical transformation gradually gives way to an undesired formation of oxygen and hydrogen. This potential above which oxygen and hydrogen are produced is termed the oxygen or hydrogen overvoltage.
In face of the aforedescribed problem, attempts have been made to provide accumulators with especially short charging times and therefore with relatively thin active mass layers upon the electrode surfaces. These layers should be held as thin as possible and the weight of the layer supports should be minimized so that the specific power (in watt-hours/kg) of the accumulator can be as large as possible.
In practice, this can be accomplished to a considerable extent in the lead-acid system by using thin foils of lead as the electrode material and transforming the surfaces of the thin lead foils by either the well-known Plante process or by other means into the active material. A storage battery or accumulator formed from such lead foils has the desired brief charging time in which complete transformation of the active mass can be effected. In this case as well the rate at which the active mass is charged is a function of the thickness of the foil and the layer formed thereof.
However, as a practical matter, such an accumulator is not technologically satisfactory since the forming process which occurs during each charge and discharge cycle, especially for the positive foil plates results in unstable active masses. During this forming process, there is a rapid transformation of the surface of the lead foils to the lead dioxide (Pb0.sub.2), while the negative foil plates undergo on long stands, a transformation to the lead sulfate (PbSO.sub.4).